1. Field of the Invention
The present invention is in the field of downhole tools used in oil and gas well drilling and downhole equipment recovery. More specifically, it is an apparatus and method for loosening a stuck object by imparting vibration to the object.
2. Background Art
In well operation, there is often a need for jarring, impact or vibration devices to move tubular objects that are stuck in a well bore, as a result, for example, of excessive friction with sand, or other materials, at a downhole location, often called “sticktion” or “stiction” force. The stuck object may be a work string, a production tube, or a drill string. It may be stuck in an unconsolidated material such as a collapsed sand formation in an open hole, by gravel pack or completion sand, or it may be stuck in a cased hole, where there is a gravel pack or completion sand between the tubular object and the casing. Additionally, it may be stuck due to failed metallic parts from downhole equipment, commonly referred to as “junk.” The terms “sand” and “soil” are used interchangeably herein, and other similar substances such as gravel and drill cuttings are intended to be included. In another instance, the object may be stuck due to a mechanical failure, such as, for example, a collapsed casing or production tubing. In yet another example, the object may be stuck due to differential sticking of the object against the borehole wall. Differential sticking is a condition whereby the drill string cannot be moved (rotated or reciprocated) along the axis of the wellbore. Differential sticking typically occurs when high-contact forces, caused by low reservoir pressures, high wellbore pressures, or both, are exerted over a sufficiently large area of the drill string.
One method employed to loosen such stuck objects is the use of an impact jar. These are typically included in a pipe or work string, near the depth at which the object is stuck in the wellbore, to provide large amplitude, uni-directional pulses or impacts of very short duration. The amplitude of the pulse is typically between 6 and 8 inches, and the duration of the pulse is typically in the range of 10 to 100 milliseconds. Impact jars are also usually single impact devices which must be recocked each time before operation, so they typically impart pulses 1 or 2 minutes apart, or in the frequency range of about 0.02 to 0.03 Hz. Therefore, only a limited amount of energy can be delivered to a stuck object over a given period of time, with this type of tool. When the object is stuck in an unconsolidated material, for example, this type of loading does not produce a favorable rate of buildup of pore pressure, in the unconsolidated material in which the tubular object is stuck. Therefore, the necessary reduction of soil strength in the area surrounding the stuck portion of the object, as a result of soil liquefaction, is not realized, and the stiction force must be overcome by a significant amount of overpull on the work string.
Some of the known impact tools require the operator to pull up on the work string with a force sufficient to pre-stress the work string, thereby providing the motive force for an impact. The impact is typically initiated when some type of valve or other triggering device in the tool triggers an action which applies the energy stored in the pre-stressed work string in the form of an impact delivered to the stuck tubular object. The force of the impact delivered by such a tool depends upon how much energy is stored in the pre-stressed work string. That is, a larger over-pull will deliver a harder blow to the stuck portion of the tubular object. Because of the aforementioned limitations of this type of tool in reducing the friction force on a tubular object stuck at a deep location, the energy put into the system in the form of overpull has to be very large, in order to overcome the stiction force between the stuck object and the unconsolidated material, and in order to mechanically break the interface bond between the tubular object and the unconsolidated material.
A second method for loosening a tubular object stuck in an unconsolidated material is the application of bi-directional, simple harmonic, vibrations of a sufficient amplitude and frequency to induce soil liquefaction, which in turn reduces the stiction force between the unconsolidated material and the tubular object. The vibration amplitude is in the range of about 0.6 inch to 0.8 inch. Rather than being discrete pulses, the vibrations are continuously applied, at a frequency of up to about 60 Hz. As compared to the use of the impact jar, this method requires much less overpull; in fact, the required overpull may be only a fraction of the original stiction force on the stuck object. Where sufficient vibration energy can be applied at the stuck location, this method can be very effective. That is, this method results in a high degree of soil liquefaction and a high degree of friction force reduction, resulting in a comparatively low extraction force requirement. However, in this method, the vibrations are commonly imparted to the tubular object at the Earth's surface, and the tool has a limited ability to propagate the vibrations to great depths in the well bore. For example, if an object is stuck in an unconsolidated material at a depth greater than that to which the removal tool can propagate sufficient vibration energy, then this method is not effective.
A third method employed to extract a stuck object in an unconsolidated material, such as soil, is delivering energy to the soil mass in the form of uni-directional pulses similar to those delivered by the impact jar, except that the pulse amplitude is much smaller, and the pulses are more closely spaced. The pulses are delivered to the tubular object near the stuck location. The pulse amplitude in this method is typically about 0.06 inch to 0.08 inch, the pulse duration is typically about 0.003 seconds, and the pulse frequency is typically about 10 to 20 Hz. Spacing the pulses more closely assists in pore pressure buildup, but the smaller pulse amplitude is generally not great enough to induce plastic strains in the soil. As a result, the degree of soil liquefaction may be only moderate, and, therefore, the resultant reduction in the stiction force also is only moderate. Because of the location of this type of tool near the stuck depth, the depth range of this method can be great, but the amount of extraction force required can still be appreciable.